Global climate change is expected to alter the genotypic composition of populations. Genotypes that are able persist in the novel environmental conditions will out compete those that cannot, affecting intra-specific genetic diversity. Previously, we found that a decade of altered precipitation patterns increased the genetic diversity of a dominant tallgrass, Andropogon gerardii, with evidence that two genotypes were being selected for in the altered conditions. Building off this research, here we address whether 1) there are detectable phenotypic differences between genotypes, and 2) if patterns of phenotypic variation match the patterns of genetic variation. We studied genotypes within the ongoing climate change experiment – the Rainfall Manipulation Plots, where half the plots receive ambient rainfall, ‘ambient treatment’, and the other half receive the same quantity of rainfall in an altered pattern, ‘altered treatment’ – and in six reference plots in adjacent prairie, ‘reference treatment’. We focused our sampling on the two most abundant genotypes in the altered and ambient treatments, and genotypes that were found among treatments. During the 2009 growing season we measured stomatal conductance, height, variation among individuals for these traits as a proxy for plasticity, and end of season biomass. We chose these measurements because they reflect tradeoffs between water-stress and ability to assimilate carbon.
Overall, stomatal conductance and height, but not end of season biomass, differed among the treatments. In the reference plots there were detectable differences between genotypes, demonstrating phenotypic differences upon which selective forces could act. In the ambient treatment, there were no detectable differences between the two most abundant genotypes for four of the measured traits, except for stomatal conductance plasticity, where the most abundant genotype had greater stomatal conductance variation among individuals. In the altered treatment, where there is greater genetic diversity, there were consistent phenotypic differences between the two most abundant genotypes. The genotypes had different stomatal conductance and height throughout the summer, with the slightly more abundant genotype having consistently greater stomatal conductance and correspondingly greater height, while the second most abundant genotype was more plastic for both stomatal conductance and height. There was no difference in end of season biomass between the genotypes. Taken together our results further support our previous finding of diversifying selection as a consequence of altered precipitation patterns. The two most abundant genotypes have consistently different phenotypes reflecting a tradeoff between mitigating water-stress and carbon assimilation and the ability to respond to more variable soil moisture conditions.